In automatic transmissions for transmitting engine power to a drive shaft via a hydraulic clutch such as a torque converter and planetary gear train, a one-way clutch comprising an overrunning clutch may for example be used. In this type of transmission, for example, a low gear ratio may be obtained when the one-way clutch restricts the rotation of one rotation element in the planetary gear train, and a high gear ratio may be obtained when the rotation element rotates in another direction which the one-way clutch allows.
In this case, when the gear ratio is changed over from high speed to low speed, the rotation direction of the rotation element changes when a load acts upon it in the opposite direction, and the one-way clutch which so far had permitted rotation of the element prevents its rotation in the reverse direction. If the rotation element and the one-way clutch collide, a severe shock may occur.
To suppress this shock, Tokkai Sho 58-207556 published by the Japanese Patent Office in 1983 and Tokkai Sho 64-85844 published by the Japanese Patent Office in 1989, disclose a method wherein an engine ignition period is delayed when there is such a gear shift so that a torque input to the converter is temporarily reduced.
However, torque suppression by retarding engine ignition timing cannot be employed when the engine running conditions are such that it is difficult to retard the ignition timing, e.g. when any further delay of the ignition timing would result in impairment of exhaust gas composition or combustion conditions.
Further in this type of controller, signal exchanges take place between a control system of the automatic transmission and that of the engine, and this requires complex hardware and software. It is also difficult to match component elements of hardware, and the cost of the controller therefore increases.